Use Option<AuthorAssociation> over String in Issue struct

[maflcko]

This is more type-safe. Also, according to the response schema (https://docs.github.com/en/rest/issues/issues?apiVersion=2022-11-28#get-an-issue), the field is not required. Only the following are required:

  "required": [
    "assignee",
    "closed_at",
    "comments",
    "comments_url",
    "events_url",
    "html_url",
    "id",
    "node_id",
    "labels",
    "labels_url",
    "milestone",
    "number",
    "repository_url",
    "state",
    "locked",
    "title",
    "url",
    "user",
    "created_at",
    "updated_at"
  ]

So, make it optional as well.

[dmgorsky]

lgtm